As it is well known, a turbomolecular pump can schematically be regarded as comprising an outer casing in which a plurality of gas pumping stages are housed.
The gas pumping stages are generally obtained through an assembly of stator rings cooperating with rotor disks that are secured to a rotatable shaft driven by a pump motor.
The pumping stages comprise a space for allowing the gas flow, known at the pumping channel, where the surfaces of the rotor disk and the facing stator are relatively spaced away, and tight zones where the surfaces of the rotor disk and the facing stator are very near to each other.
The rotor disks can be either flat (plane) disks or disks that are provided with closely spaced apart inclined blades.
A vacuum pump of the turbomolecular type comprises both flat disks and bladed disks, and is capable of achieving pressure levels as low as 10.sup.-8 Pa.
In order to reach the above vacuum levels with the presently used pumps, the rotor must rotate at a speed as high as 100,000 rpm.
The vacuum pumps such as turbomolecular pumps is used in the field of Integrated Circuits (ICs) manufacturing. In the manufacturing cycle of ICs there are used gas mixtures such as HCl, Hbr, Cl.sub.2, Fl.sub.2, NH.sub.3, etc. that are well-known as highly corrosive gases.
One of the main problems when using turbomolecular pumps in the ICs manufacturing industry, is due to the accumulation of a certain amount of gas because of the flow through the pumping stages.
As a consequence, the surfaces of the internal components of the pump, particularly the rotor surface, come into direct contact with such gas mixtures and are subjected to the corrosive action thereof.
There are also known rotors for turbomolecular pumps provided with a metal or ceramic coating as a protection against the action of such corrosive gases.
The known protective metal coating is generally applied to the rotor by means of nickel-plating, zinc plating or anodizing processes.
As already mentioned, the rotor of a turbomolecular pump is rotated at very high speeds, usually not lower than 25,000 rpm. Due to the very high rotation speed of the rotor and to the extremely reduced gap between the pump rotor and the stator in the pumping stages, a mass distribution in the rotor body that is not homogeneous with respect to its axis of rotation can cause a force unbalance which jeopardizes the working of the pump up to a failure of its components.
Thus, an essential requirement in manufacturing a turbomolecular pump, particularly to be used with corrosive gases, is to achieve a substantially perfect rotational balance of the rotor body.
The known metal or ceramic coatings used until now have the drawback of being unsuitable for application onto objects that are to remain perfectly balanced while maintaining very smooth surfaces such as the rotor of a turbomolecular pump. Due to the complex geometrical shape and the small size of the areas in which the blades are attached to the rotor, the thickness of the metal or ceramic coating may be sufficient and easily corroded.
In order to prevent this from happening, the amount of the protective material deposited onto the rotor body it is often increased, but this countermeasure can lead to a non-uniform thickness of the protection coating of the flat surfaces of the rotor disks that sometimes results in being too thick.
Consequently, an additional finishing step becomes necessary in order to level the surfaces on which the deposited material has a not uniform thickness.
The object of the present invention is to overcome the above-mentioned drawbacks by providing a rotor for a vacuum pump which is corrosion resistant while at the same time constructed easily and inexpensively.